SpaceSC — Use Case Build Tree

Tier model. Tier 1 = Sources (SKR + Pinecone) · Tier 2 = Insights (Insight Ledger SQLite) · Tier 3 = Research Notes (NEW, §8.4a.17) · Tier-3-pending = Open Questions (NEW Notion DB, §8.4a.17).
Research Session is now a horizontal pattern across UC1–UC4 (§8.4a.17). Shared backend (W1–W6); per-UC chat surfaces in W7 (UC1) and §8.4a.18–.20 (UC2–UC4).